The present invention relates to a process for removing polychlorinated biphenyls (hereinafter referred to as "PCB") from transformer-insulating liquids. The invention has special significance in removing residual PCB from transformer-insulating liquids, particularly silicon oils, which have been substituted for PCB oils previously used as a transformer-insulating liquid.
A principal field of application for PCB is their application as insulating liquids or coolants. PCB is a so-called askarel, meaning they are electrically insulating, flame-resistant liquids that generate neither combustible nor explosive gases in electric arcs. These askarels find wide application as transformer-insulating liquids.
To protect the environment, and for reasons of safety, it has proved necessary to adapt PCB-cooled transformers to insulating liquids or coolants that are ecologically less hazardous, particularly silicon oils. However, it has become apparent that it is by no means easy to substitute other transformer-insulating liquids, for example silicon oils, for PCB oils, because it is practically impossible to remove PCB oil from a transformer without leaving residual PCB, even when the transformer, after transport, is disassembled and thoroughly cleaned at an appropriate transfer facility. Over time, the residual quantities of PCB oil that inevitably remain in the transformers and/or their components enter the new transformer-insulating liquid so that, after some time, contents of up to 10% by volume of PCB oil are present in the replacement transformer-insulating liquid.
It is common practice to remove these PCB contaminants from the replacement transformer-insulating liquid by adsorption or chemical conversion in special filters. However, substantially complete removal of PCB by such known methods takes a long period of time to achieve.